1. Field of the Invention
The present invention relates generally to the art of multilayered films or coatings providing high transmittance and low emissivity, to articles coated with such films or coatings, and more particularly to such coatings or films formed of metal and metal oxides and deposited on transparent substrates.
2. Discussion of the Presently Available Technology
High transmittance, low emissivity films or coatings generally include a reflective metal film or layer which provides infrared reflectance and low emissivity, sandwiched between dielectric, antireflective films or layers of metal oxides to reduce the visible reflectance. These multilayer coatings are typically produced by cathode sputtering, especially magnetron sputtering.
U.S. Pat. No. 4,610,771 to Gillery provides a film composition of an oxide of a zinc-tin alloy, as well as multiple-layer films of silver and zinc-tin alloy oxide layers for use as a high transmittance, low emissivity coating. This oxide film may have the composition of zinc stannate (Zn2SnO4), but may also range from that exact composition.
U.S. Pat. No. 4,806,220 to Finley discloses a multilayer film coating suitable for high temperature processing. A type of this coating utilizes metal primer layers e.g. titanium primer layer both above and below a reflective metal layer of greater thickness than usual, up to 50 Angstroms in thickness.
It would be desirable to produce high transmittance films and article coated with such films that have minimal emissivity, low electrical resistivity and improved shear resistance, which exhibit improved resistance to weathering and can withstand high temperature processing where the use of a titanium primer layer below the reflective metal layer may be avoided. Alternatively, in the case where more than one reflective metal layer is present, it would be desirable to avoid the use of a titanium primer adjacent the substrate-near side of any of the reflective metal layers.
The present invention is directed to multilayer high transmittance, low emissivity coatings on transparent substrates which feature an antireflective base film of at least two parts on the substrate-near side of a metallic, reflective film, that is to say the side of the metallic reflective film that is in parallel facing relationship with the substrate. A first of the two parts is in contact with the metallic film. This first film-part has crystalline properties for causing the metallic film to deposit in a low resistivity structure. The second of the two film-parts supports the first part and is of a chemically and thermally more durable, preferably amorphous, material. The present invention includes both coatings having a single metallic reflective film and coatings with multiple metallic, reflective films, in which case the novel base film of the present invention can be utilized for just one of the multiple metallic films, for several or for all of them.
More particularly the present invention is directed to a high transmittance, low emissivity coated article having:
a transparent, nonmetallic substrate;
a dielectric, antireflective base film deposited over the substrate, the base film including a crystalline metal-contacting film-part and a support film-part where the support film-part is over and may be in contact with the substrate and wherein the support film-part includes a material other than the crystalline metal-contacting film-part;
a metallic reflective film deposited on the crystalline metal-contacting film-part of the base film;
a primer film deposited on the metallic reflective film; and
a dielectric, antireflective film deposited on the primer film.
In an alternative embodiment of the present invention, an exterior protective overcoat layer is deposited on the dielectric, antireflective film.
In a preferred embodiment of the present invention, the transparent, nonmetallic substrate is glass, the support film-part is a zinc stannate film, the crystalline metal-contacting film-part is a zinc oxide film; the metallic reflective film is a silver film, the primer film is deposited as titanium metal, the dielectric, antireflective film is a zinc stannate film, and the exterior protective overcoat layer is a titanium oxide film.
Coated articles of the invention also feature, in combination with the above-mentioned base film or independently thereof, a newly discovered, particularly advantageous subrange of the thicker primer layers, or films, of the above-referenced U.S. Pat. No. 4,806,220 to Finley.
Nomenclature and Measurement Techniques
When referring to crystal planes herein, representation of the planar indices within braces, i.e. { }, is a reference to all planes of that form. This convention is explained, for instance, in Cullity""s xe2x80x9cElements of X-Ray Diffractionxe2x80x9d, Addison-Wesley, 1956, pages 37-42.
Gas percentages herein are on a flow (volume/unit time, SCCM) basis.
Disclosed thicknesses of the various layers of the multiple layered coatings of the present invention herein are determined on the basis of two different procedures, depending on whether layer is a dielectric layer or a metal layer.
The thickness of dielectric layers, or films, is determined by the aid of a commercial stylus profiler (hereinafter referred to as the xe2x80x9cStylus Methodxe2x80x9d), as follows. Before the deposition of each layer, a narrow line is drawn on the glass substrate with an acetone soluble ink. Following the deposition of the coating the line, and that portion of the coating deposited over it, is removed by wetting the surface with acetone and gently wiping with laboratory tissue. This creates a well-defined step on the surface of the glass whose height is equal to the thickness of the layer and can be measured with a profiler.
Two potential complications make the Stylus Method used for measuring the thickness of dielectric layers less favorable for measuring the thickness of thin metal films. First, metals, such as titanium and silver, are more prone to abrasion when wiped. Second, metals react readily with the ambient atmosphere when removed from a vacuum chamber. Both of these phenomena can result in significant errors if thickness measurements are made via the Stylus Method.
As an alternative, a method that will be referred to as the xe2x80x9cXRF Methodxe2x80x9d herein is used to measure the thickness of metal layers. The XRF method uses a calibrated x-ray fluorescence instrument to measure the weight of the metal per unit area of the coating (namely, in xcexcg/cm2). The XRF method makes the assumption that the metal film is as dense as its bulk form. With this assumption, the metal film""s measured weight per unit area is then converted to a thickness in Angstroms, using its bulk density.
For completeness sake, it should be noted that sputtered metal films are often less dense than their corresponding bulk metals, so that above described assumption is not always precisely correct, and the XRF Method may in some cases underestimate the thickness of the metal film due to this variation in density. Thus, for the thin metal films, the initial measurement of weight per unit area (xcexcg/cm2) is more accurate than the corresponding conversion to thickness based upon bulk density. Nonetheless, the XRF Method provides a useful approximation for comparing the relative thicknesses of the layers in a coating. Thickness tolerances given herein represent twice the standard deviation of the measurements.